Preheat oven to 350 degrees. Butter a 13x9 inch baking pan. Melt butter in a medium saucepan. Add brown sugar and cook, stirring, until bubbly. Transfer mixture to a large bowl and cool 10 minutes. Beat in eggs and vanilla. Stir in flour, baking powder and salt. Fold in pecans and chocolate chips. Pour batter into prepared pan and bake 25 to 30 minutes until a toothpick comes out clean. Transfer pan to a rack and cool completely. Cut into bars and store in an airtight container at room temperature.